In 1966, John Lennon went to a London gallery. The exhibit there was by an artist named Yoko Ono, and one of the pieces was a ladder, a magnifying glass and a tiny word written on the ceiling.

The word – Lennon found out to his delight and relief – was “YES.” And once he read it, he knew he had to meet whoever had painted it.

Sometimes though the artist has to say NO.

Ai Weiwei, for example, has been saying No for years. No to an Olympics that evicted people from their own city. To a government that refuses to investigate lethally shoddy school construction. To police who try to cover up their own brutality.

As “Ai Weiwei: Never Sorry” shows, this does not make him very popular with the Chinese government.

Unfortunately for them, Ai Weiwei is very popular around the world, at least among art lovers. The “Bird’s Nest” Olympic stadium he helped design grabbed global praise. His Circle of Animals/Zodiac Heads sculptures come to Princeton for a year, starting in August.

And this is Beijing’s problem. Ai is too well-known to be quietly silenced, and too dangerous to be allowed to speak.

But, as this sometimes haphazard documentary shows, Ai won’t stop talking. Or blogging. Or tweeting. Or trying to take photos and film of everything, even of the policemen filming him.

To an authoritarian culture which values order and respect, it is all very disordered and embarrassing (although it’s unlikely anything incensed the regime more than hearing Ai later bitterly denounce the Olympics he’d worked on).

Ai’s demands, such as they are – he is a conceptual artist, not a politician – are the ones reasonable people make of any dictatorship. More democracy. More freedom. And, of course, they are exactly the sort of reasonable requests that make every dictatorship nervous.

But China may have reason to worry. When Ai shows up at a cheap outdoor restaurant, he’s soon surrounded by fans. When a crisis looms, thousands of ordinary Chinese people reach out to help him.

He’s a charismatic figure with a rough, modern edge, a decent command of English and a taste for the Carnegie Deli’s corned beef. (He lived in New York for years -- undoubtedly where he picked up his fondness for literally flipping his middle finger at hated instutions.) You can see why he’s become such a symbol.

The movie has some trouble explaining him as a man, though. Things seem to come out of context; even when other reporters show up for interviews, director Alison Klayman still holds back. Simple questions about Ai’s money and marriage aren’t asked early on – a problem because the answers become very pertinent by the film’s end.

But that doesn’t detract from the impish charm of this chubby, mischievous bohemian who named one of his early shows – well never mind what he named it, because I can’t print it here. But he named another one, “So Sorry” – a sardonic nod to the patronizing, pidgin-English cliché.

And a lie too, of course. Because, as the film notes, Ai Weiwei isn’t sorry at all. Not one bit.

Ratings note: The film contains strong language, brief violence and some nude images.

'Ai Weiwei: Never Sorry' (R) Sundance Selects (91 min.)
Directed by Alison Klayman. In Mandarin and English, with English subtitles. Now playing in New York.
THREE STARS